There are many designs of water heaters in which combustion products from a burner are directed to flow in heat exchange relationship to a water tank. One example is shown in U.S. Pat. No. 5,361,729 owned by the present applicants in which combustion products from a burner below the tank are directed to flow up one side of the tank, across the top, and down the other side(s) and thence to an external balanced flue. Although very effective in operation, this water heater requires relatively complex sheet metal components particularly to define the flow paths for the combustion products up one side and down the other side of the tank.
Another water heater is shown in patent specification AU-38573/93 in which combustion products are directed up through a central flue extending centrally through the height of the tank. Complex baffle arrangements are used to try to increase the efficiency of extraction of heat from the combustion products. Also there are problems with sealing the flue at the top and bottom of the tank.
In water heaters where water is heated in a storage vessel or tank and from which water is to be drawn at a temperature within a predetermined range, it is usual to provide a thermostat probe which extends into the tank to sense the temperature of the water in the tank. If the temperature of the water in the tank falls, e.g. due to progressive loss of heat through the tank to the ambient surroundings, or due to tapping of water from the tank and the consequent introduction of replacement cold water into the tank, the temperature probe senses the fall in temperature and initiates operation of heating means associated with the tank. In the case of a water heater where the heating means comprises a burner for burning of fuel, the heat from which is used to raise the temperature of water in the tank, the sensing of the temperature drop by the temperature probe initiates ignition of the burner. When the water temperature in the tank is raised to a predetermined level as sensed by the temperature probe, the burner is extinguished, by discontinuing supply of fuel.
The lower temperature at which the burner is ignited to heat water in the tank, and the upper temperature at which the burner is extinguished is a function of the construction and operation of the temperature probe. A known and conventional temperature probe used in gas water heaters for domestic and commercial hot water supply comprises a bimetallic rod made of the alloy Invar, a stainless steel alloy. The rod is manufactured to a close tolerance in length and is mounted in association with a gas supply valve so that the outer end of the rod pushes on an operative component of the gas valve to cause it to open when the temperature of water into which the inner end of the rod extends falls to a predetermined level. When the temperature of the water in the tank rises to an upper limit the rod causes the gas valve to close off supply of gas to the burner. The difference between the lower temperature at which the valve is opened and the upper temperature at which the valve is closed is called the "differential" for the particular temperature probe. Commonly available temperature probes for operating gas valves of commercial or domestic water heaters have a differential of 14.degree. C. and this value is prescribed as the maximum differential allowable in regulations of the Australian Gas Association (the regulatory body for setting down and monitoring compliance with standards for gas appliances for the Australian market).
A water heater with a temperature probe and gas valve operating at the maximum allowed differential of 14.degree. C. may be intended to supply hot water at a maximum of, say, 70.degree. C. (which may be diluted to, say, 50.degree. C. by mixing with cold water from the mains supply after the water at 70.degree. C. is drawn from the tank). However with a differential of 14.degree. C., water in the tank can fall to 56.degree. C. before the burner is operated to raise the temperature again. This means that a consumer can find that the domestic hot water supply system when called upon to supply hot water, may do so at a temperature as low as 56.degree. C. (which the downstream dilution may reduce to a temperature in the low 40.degree. C. range)--an unacceptably low temperature for domestic requirements such as showering, bathing, washing dishes, or clothes, washing machines.
Therefore in use it is desirable to have a temperature differential substantially less than 14.degree. C. However it is difficult to obtain commercially temperature probes for use with gas valves for hot water services having temperature differentials substantially less than 14.degree. C. The present applicant sources a special thermostat with a guaranteed differential of 11.degree. C. for use in its domestic and commercial gas hot water systems and this differential of 11.degree. C. is about the best that is being offered in the market. However even a guaranteed differential of 11.degree. C. can produce markedly different temperatures of water being supplied to the user of the hot water system depending on whether the water in the tank happened to be towards the lower end of the temperature range or towards the upper end of the temperature range determined by the differential of the temperature probe being used.